Monopropellant comprising a difluoraminoalkane and fuming nitric acid



United States Fatent (j 3,310,444 MONOPROPELLANT COMPRISING A DI- FLUORAMINOALKANE AND FUMING V NITRIC ACID Jack R. Gould, Monsey, and Edward M. Butler, Yonkers, N.Y., assignors to Stauflier Chemical Company, New York, N.Y., a corporation of Delaware No Drawing. Filed Apr. 9, 1962, Ser. No. 186,269 8 Claims. (Cl. 149-74) This invention relates to liquid monopropellants having negligible sensitivity to adiabatic compression, and which can be stored and handled under substantially ambient conditions. The invention is particularly concerned with high energy liquid monopropellant compositions including a method of preparing them in which fuming nitric acid is utilized as an auxiliary oxidizing agent to supplement the combustion of an organodifluoramino monopropellant compound for the purpose of attaining higher specific impulse.

The term monopropellant composition as understood herein, refers to a composition containing in admixture a fuel and an oxidizer and capable of exothermic decomposition in the absence of an oxidizing atmosphere into essentially gaseous exhaust products. A monopropellant compound as used herein, refers to a single compound, the chemical structure of which embodies both fuel and oxidizer elements and which is capable of undergoing exothermic decomposition as above defined, but without the need of an auxiliary oxidizer. A monopropellant may thus be a single compound or it may be a composition comprising a mixture of several ingredients.

It is recognized in the art that a liquid monopropellant offers several advantages for use in rocket engines of the type which derive their thrust from the combustion of liquid propellants. One of the most outstanding features of a monopropellant liquid is the fact it does away with the need for multicomponent liquid propellant systems commonly used in liquid fuel rocket engines. By employing only a single liquid component, construction of the rocket engine and its pumping and fuel storage and distribution systems is thereby greatly simplified and reduced in weight, with concomitant great increase in engine reliability and vehicle range.

Although the advantages of liquid monopropellants are manifest, their formulation presents certain practical difficulties to the art. The major problems are to provide a propellant which is storable for long periods of time as a liquid at ambient temperatures; which possesses low sensitivity to mechanical and thermal shock; which is incapable of propagating a detonation under conditions encountered in use in rocket engines and auxiliary components; which possesses good thermal stability as defined by standards of those skilled in the art; which yields a high specific impulse (i.e., I 280 lb.-sec./1b. at P =1000 p.s.i.a.) upon self-combustion in a rocket engine; and which possesses an array of other desirable physical properties which permit its safe, reliable and facile use under a variety of field conditions.

It has now been discovered that storable, liquid high energy monopropellants can be formulated by mixing a liquid organodifiuoramino compound with a strong oxidizing agent of the type exemplified by fuming nitric acid and the provision of such monopropellant-s constitutes the primary object and purpose of this invention. It is also an important object of the invention to provide methods of preparing the aforesaid monopropellants. Other objects and purposes will become manifest as the description proceeds.

Our invention is predicated on the discovery that certain liquid organodifluoramino compounds can be mixed with a fuming nitric acid oxidizer to yield high energy liquid monopropellants. In addition to providing high specific impulse (I 280 lb.-sec./lb. at P 1000 p.s.i.a.), our monopropellants possess low sensitivity to impact and adiabatic compession, and exhibit good thermal stability. By virtue of the afore enumerated physical properties, the monopropellants as herein described combine high energy normally associated with bi-propellant systems with the convenience and handling which previously was possible only in the case of low-energy monopropellant fuel systems. By thus providing the art with a reliable and high energy monopropellant, it becomes .possible to design rocket craft equipped with simpler engines with concomitant increases in reliability and performance.

It is to be pointed out the organodifiuoramino compounds are in themselves monopropellants since they embody in the same molecular structure both the fuel and oxidizer elements, these being hydrogen and fluorine respectively. However, when the aforesaid compounds are used alone as monopropellants, the resulting specific impulse is too low to make them practical as primary propellants in rocket craft. While undergoing self-combustion, the carbon of the difluoramino compound is not oxidized but appears in the exhaust products as elemental carbon. As a consequence, self-combustion of these monopropellant compounds does not result in a particularly high specific impulse, i.e., 27O lb./-sec./lb. at P =1000 p.s.i.a. However, by utilizing an auxiliary oxidizing agent as exemplified by fuming nitric acid in admixture with the organodifluroamino compound we have succeeded in effecting oxidation of the carbon to oxides of carbon when such monopropellant compositions are subjected to self-combustion. By thus augmenting the oxidizing elements of the organodifluoramino compound with fuming nitric acid a monopropellant composition is thus produced having considerably greater specific impulse; in fact the specific impulse of certain of these blends falls in the neighborhood of 290 seconds.

The table below contains a listing of specific impulses which can be obtained from the monopropellant com positions as described herein.

The monopropellants of this invention are predicated upon the discovery that the stoichiometric proportions of fuming nitric acids and difiuoramino compounds, as generally exemplified by difluoraminoalkanes, can be formulated to give, upon combustion, maximum heat re lease with minimum average molecular weight combustion products. The aforesaid concepts can be schematically illustrated by referring to the following equations:

The monopropellant compounds used in formulating the monopropellants of the invention are difluoraminoalkanes having at least one -NF group and wherein the alkane chain or bridge may be substituted by fluorine and lower alkyl radicals of from 1 to 10 carbon atoms. Ex-

3,310,444 .Prt e Mstr 251 72..

3 emplary difluoraminoalkanes, falling within the scope of the aforesaid description, include the following:

TABLE Calculated Specific Impulse Composition of lllonopropellant (lb.-sec./lb. at

Pch =l,000 p.s.i.a., Pex=14.7 p.s.i.a.)

1.0 NFzCIIaCHzNFz plus 1.333 HNO; (WFNA) 1 288 1.0 NFzCl-IzC (NFQCH; plus 2.0 HNOs (WFNA 282 1.0 NF2CH2CH(NF2)CH2GHs plus 2.667 HNOa (WFNA) 275 1.0 NF2CI'I2CH(NFz)CH2NFz plus 2.0 HNOa (WFNA) 285 4.0 NF CI-IQCIHNFQCH NFZ plus 1.0 NF2CH2CH(NF2) CHzCHs plus 10.67 I-INOs (WFNA) 284 2.0 NF2C}I2CH(NF2) CHzNFz plus 1.0 NFzCHzCI-HNFQCH; plus 6.0 HN 03 (WFN A) 285 0.863 CzH (NF2)2 plus (1.0 I'INOK plus 0.226 N02) (RFNA) 2 289 0.575 NFzCH2OI-I(NF )OH3 plus (1.0 HNOa plus 0.226 N02) (RFNA) 284 0.575 NFZCI-IQCH(NF2)CHZNF2 plus (1.0 HNO: plus 0226 N02) (RFNA) 286 That it should be possible to mix the highly reactive entity of the type exemplified by organodifiuoramino compounds with a vigorous oxidizing agent such as a fuming nitric acid is indeed surprising and unexpected. In fact, it would be predicted that the combining of such active entities would produce an immediate hypergolic reaction. We have found that these organodifiuoramino compounds when mixed with fuming nitric acid actually result in stable mixtures practically usable as monopropellants. In fact we have ascertained that our monopropellants even though mixtures of two highly reactive entities, exhibit lower sensitivity to adiabatic compression than the single monopropellant components, namely, the organodifiuoramino compounds, and this finding constitutes another unexpected feature of our invention. We have for instance, prepared mixtures of bisdifluoraminopropane and nitric acid, and allowed such mixtures to stand several days at room temperature. During these stability tests we found no evidence of chemical interaction of the type as would be evidenced by heat release, gas evolution, etc. We also examined our monopropellant compositions by means of infrared analysis with a view to detect any evidence of breakdown. For example, we prepared mixtures of difiuoraminopropane and fuming nitric acid and recorded the infrared spectra of such mixtures. After standing for a prolonged period of time such mixtures were again subjected to infrared analysis. An examination of the spectra before and after standing revealed no substantial or detectable change. Such tests clearly establish the storage stability of our new monopropellants.

The high energy monopropellants as contemplated herein can be readily prepared. In general we have ascertained that excellent results are obtained by mixing the requisite organodifiuoramino alkane with the turning nitric acid at room temperature. The components are completely miscible with production of a clear liquid, the color of which depends on the type of fuming nitric acid employed. As previously pointed out, no evidence of chemical interaction or heat release can be detected. It is of course, necessary that the ratio of organodifiuoramino compound and fuming nitric acid be in such molar ratios that the carbon elements of the difiuoramino compound are effectively oxidized during the combustion process in order to yield the desired exhaust products, i.e., oxides of carbon, nitrogen, hydrogen fluoride, and hydrogen. Although the type of fuming nitric acid can be varied, the composition of the exhaust products can be held constant by simple adjustments in ratios of the monopropellant components. Varying the ratio of oxidizing agent to the organodifiuoramino compound affects the specific impulse although in general high impulses are obtained, i.e., 280 to 290 seconds, provided that the carbon of the organodifiuoramino compound is converted completely to its oxides. For example, by using one mole of bisdifluoraminopropane and two moles of white fuming nitric acid there is produced a system whose specific impulse is 282.1 seconds. On the other hand, if red fuming nitric acid is the acid used (contains 14-' -l.0% by weight N0 and in such proportions that all of the carbon in the organodifiuoramino component is converted to carbon dioxide, a mixture having a specific impulse of 283.7 seconds is produced. The combustion products in both instances are essentially the same; carbon dioxide, nitrogen, hydrogen fluoride and hydrogen.

The difiuoramino alkanes used in formulating the herein described monopropellants are prepared by reacting a slight molar excess of tetrafiuorohydrazine with an unsaturated organic compound as exemplified by ethylene, propylene, butene, etc. Typically, bisdifluoraminopropane used is prepared by reacting one mole of propylene with a slight molar excess of tetrafluorohydrazine in the gas phase at 200 C. for 1-2 minutes and isolating the resultant bisdifluoraminopropane by a simple one-plate distillation.

The fuming nitric acids which constitute the other essential component of our monopropellant compositions are known entities the description and properties of which can be obtained by consulting the technical chemical literature. WFNA (white fuming nitric acid) is essentially HNO and is also named as Type I fuming nitric acid and RFNA (red fuming nitric acid) is essentially 82-85% HNO +l4- -l% N0 and is also named as Type III fuming nitric acid.

The following examples are inserted for the purpose of illustrating the invention in greater detail. However, as those skilled in the art will appreciate, various modifications and ramifications of the invention can be practiced without departing from the scope or spirit thereof.

Example 1 Bisdifluoraminopropane+WFNA (mole ratio To a sample of 0.3416 g. of bisdifluoraminopropane was added 0.2940 g. of 100% HNO (WFNA). Immediate solution occurred, with no chemical interaction evidenced by heat release, gas evolution, etc. Examination of the infrared spectrum of this solution revealed that bisdifluoraminopropane was present, substantially unchanged. Upon standing for many days at room temperature no change in the. composition could be detected. This new monopropellant is a clear, yellow liquid.

The drop-weight sensitivity (Olin Mathieson Drop Weight Tester) of this composition was determined to be 3040 kg.-cm. This indicates remarkably low sensitivity for a monopropellant of this energy content (I 282 seconds at P =l000 p.s.i.a., P =l4.7 p.s.i.a.). Detonation of this composition could not be initiated by hammer blows upon a sample placed upon a steel plate. Adiabatic compression testing of this monopropellant also revealed it to be remarkably insensitive. A compression sensitivity value of 21 kg.-cm./ml. was obtained for this mixture (JANAF Test Method No. 5recommended by the Joint Army-Navy-Air Force Panel on Liquid Propellant Test Methods). This value is compared to values of 6.7 for n-propyl nitrate and 10.4 for nitromethane, both low energy monopropellants considered to be extremely insensitive. The thermal stability or self heat temperature- 5 Example Bisdifiuoraminopropane+RFNA (mole ratio 0.575 C H N F 1.0 HNO /0.226 N To a 0.85 g. sample of bisdifluoraminopropane was added 0.74 g. of RFNA (14% N0 by weight). Complete solution occurred, with no evidence of chemical inter-action, giving a clear yellow liquid monopropellant which could not be frozen at 90 C., nor detonated at 120 kg.-cm. using the same apparatus mentioned in Example 1, nor detonated by hammer blows.

Ignition of this high energy monopropellant was readily induced with a flame. It burned rapidly leaving no carbonaceous deposits. Its theoretical specific impulse is 284 seconds at P =1000 p.s.i.a., P =l4.7 p.s.i.a.

A compression sensitivity value of 50 kg.-cm./ml. (JANAF Test Method No. and a self-heat temperature of 270 F. (JANAF Test No. 6) was obtained for this mixture. This mixture is almost 5 times less compressionsensitive than nitrometh-ane and times less sensitive than n-propyl nitrate.

Example 3 Bisdifluoraminobutane-I-WFNA (mole ratio 1.0 C H N F 2.667 HNO WFNA was added to a sample of bisdifiuoraminobutane in the above stated mole ratio. Immediate solution occurred with no evidence of interaction to give a clear, yellow liquid monopropellant, theoretical I =275 seconds at P =1000 p.s.i.a. and P =l4.7 p.s.i.a. The impact (drop-weight) sensitivity of this new composition was found to be above kg.-cm.

Example 4 Bisdifiuoraminoethane+RFNA (mole ratio 0.863 C H N F 1.0 HNO 0.226 N0 To a sample of 0.767 g. of bisdifluoraminoethane was added 0.495 g. of RFNA. Immediate solution occurred, with no chemical interaction evidenced by heat release, gas evolution, etc. After standing for 3 weeks at ambient temperature, in glass, the sample showed no signs of deterioration. The dropweig-ht (Olin Mathieson apparatus) value for this mixture was found to be 110 kg.-cm. The 1 of this system (P =1000 p.s.i.a., P 14.7 p.s.i.a.) is 289 seconds.

Example 5 Trisdifluoraminopropane-l-RFNA (mole ratio 0.776 C H N F /135 HNO /0.305 N0 To a 0.383 g. sample of trisdifluoraminopropane was added 0.25 g. of RFNA 14% N0 by weight). A homogeneous, clear liquid resulted which exhibited the color typical of RFNA.

f; v No evidence of interaction was noted. Using the same drop-weight sensitivity apparatus mentioned in Example 1, this high energy monopropellant composition could not be detonated at kg.-cm. at ambient temperatures; whereas the neat trisdifluoraminopropane component did detonate at 10 kg.-cm. in the same apparatus under essentially the same conditions. The theoretical specific impulse of this monopropellant composition is 286 seconds at P 1000 p.s.i.a., P =14.7 p.s.i.a. and T =3655 K.

We claim: I

1. A storable high energy homogeneous liquid monopr-opellant composition characterized by low impact sensitivity consisting essentially of a substantially non-high explosive organic monopropellant wherein the said monopropellant is a difluoraminoalk-ane having at least one -NF group the length of the alkane chain being from 1 to 10 carbon atoms plus a supplementary oxidizing agent consisting essentially of a fuming nitric acid of low water content, the oxidizing agent being sufficient to permit the carbon of the monopropellant to be converted into essentially gaseous oxides of carbon during combustion.

2. The composition according to claim 1 wherein the oxidizing agent is white fuming nitric acid.

3. The composition according to claim 1 wherein the oxidizing agent is red fuming nitric acid.

4. The composition according to claim 1 wherein the oxidizing agent is white fuming nitric acid and the difluoramine is bisdifluoraminopropane.

5. The composition according to claim 1 wherein the oxidizing agent is red fuming nitric acid and the difluoramine compound is bisdifluoraminopropane.

6. The composition according to claim 1 wherein the oxidizing agent is white fuming nitric acid and the difiuoramine compound is bisdifluoraminobutane.

7. The composition according to claim 1 wherein the oxidizing agent is red fuming nitric acid and the difiuoramine compound is bisdifluoraminoethane.

8. The composition according to claim 1 wherein the oxidizing agent is red fuming nitric acid and the difluoramine compound is trisdifluoraminopropane.

References Cited by the Examiner UNITED STATES PATENTS 1/1965 Frazer 260-583 Disclaimer 3,310,444.Jack R. Gould, Monseg and Edward M. Butler, Yonkers, N.Y

MON OPROPELLANT MPRISING A DIFLUORAMINOAL:

KANE AND FUMING NITRIC ACID. Patent dated Mar. 21, 1967. Disclaimer filed May 11, 1970, by the assignee, Staufier Chemical Uompany.

Hereby enters this disclaimer to claims 1, 2, and 6 of said patent.

[Ofiicial Gazette June 23, 1970.] 

1. A STORABLE HIGH ENERGY HOMOGENOUS LIQUID MONOPROPELLANT COMPOSITIN CHARACTERIZED BY LOW IMPACT SENSITIVITY CONSISTING ESSENTIALLY OF A SUBSTANTIALLY NON-HIGH EXPLOSIVE ORGANIC MONOPROPELLANT WHEREIN THE SAID MONOPROPELLANT IS A DIFLUORAMINOALKANE HAVING AT LEAST ONE -NF2 GROUP THE LENGTH OF THE ALKANE CHAIN BEING FROM 1 TO 10 CARBON ATOMS PLUS A SUPPLEMENTARY OXIDIZING AGENT CONSISTING ESSENTIALLY OF A FUMING NITRIC ACID OF LOW WATER CONTENT, THE OXIDIZING AGENT BEING SUFFICIENT TO PERMIT THE CARBON OF THE MONOPROPELLANT TO BE CONVERTED INTO ESSENTIALLY GASEOUS OXIDES OF CARBON DURING COMBUSTION. 